Transitions: The Evolution of Life

The above is a picture of a new fossil, called Tiktaalik roseae, that was recently discovered on Canada. The fossil is important because it fills in a gap in the transition from fish to amphibians and provides clues as to how the transition took place. The picture below is of a Coelacanth

Coelacanths are often called living fossils. They are generally considered to be somehow related to the groups that gave rise to tetrapods. In reality this is somewhat inaccurate. Coelacanths are actually highly specialized derivatives of groups common in the Paleozoic and Mesozoic eras. In particular, they are sarcopterygians. Another name for them are lobe finned fishes (as opposed to ray finned). The lobe fins seem to be an adaptation to life on or near the bottom where they can be used to push against the bottom. Because of this research has focused on their relationship to early tetrapods. Over the years a number of important fossils have been discovered that are important to the issue.

Panderichthys - the fish second from the bottom in the above picture - dates to about 385 million years ago. Prior to the discovery of Tiktaalik roseae the earliest tetrapods dated to about 376 million years ago. Tiktaalik roseae dates to somewhere around 382 million years ago. Why is this important? Ahlberg and Clack, in thier recent Nature commentary, explain it the best:

The gap was bounded at the top by primitive Devonian tetrapods such as Ichthyostega and Acanthostega from Greenland, and at the bottom by Panderichthys, a tetrapod-like predatory fish from the latest Middle Devonian of Latvia … Ichthyostega…and Acanthostega5 retain true fish tails with fin rays but are nevertheless unambiguous
tetrapods with limbs that bear digits… Panderichthys… is vaguely crocodile-shaped and, unlike the rather conventional osteolepiform fishes farther down the tree, looks like afish–tetrapod transitional form. The shape of the pectoral fin skeleton and shoulder girdle are intermediate between those of osteolepiforms and tetrapods, suggesting that Panderichthys was beginning to ‘walk’, but perhaps in shallow water rather than on land…

Into this drops Tiktaalik roseae. Like Panderichthys, Tiktaalik has pelvic fin rays, retain fin rays in paired appendages and has well developed gill arches. On the other hand, Tiktaalik is more tetrapodlike in its feeding and breathing apparatus:

These changes probably relate to breathing and feeding, which are linked in fishes because the movements used for gill ventilation can also be used to suck food into the mouth. A longer snout suggests a shift from sucking towards snapping up prey, whereas the loss of the gill cover bones (which turned the gill cover into a soft flap) probably correlates with reduced water flow through the gill chamber.

Tiktaalik also has some interesting features of its postcranial anatomy which link it to later tetrapods. Especially in its fins. The fins are adapted to flex gently upwards - as if the fin were being used to support the body. One of the interesting differences between fins and Tiktaalik limbs is that the later contain bones that comprise mobile wrist and ankles:

Another interesting feature is the central axis formed by the some of the long bones (red arrow in the picture below):

As Shubin et al point out in their article on the pectoral fin of Tiktaalik:

A fin axis that extends distal to the ulnare has been unknown in any tetrapodomorph… until the discovery of Tiktaalik. As in porolepiforms and dipnoans, the axis of Tiktaalik lies in the centre of the fin. If the five radials of Tiktaalik are homologous to digital rays, then the axis of the tetrapod limb would extend from the humerus through digit three. Unfortunately, the absence of a well-defined axis in other tetrapodomorphs leaves uncertain whether a central axis is primitive for tetrapods or if it evolved separately in Tiktaalik. Testing these competing hypotheses awaits the discovery of other tetrapodomorph fins with axes that project into the distal
fin.
The pectoral skeleton of Tiktaalik is transitional between fish
fin and tetrapod limb. Comparison of the fin with those of related
fish reveals that the manus is not a de novo novelty of tetrapods;
rather, it was assembled in fishes over evolutionary time to meet
the diverse challenges of life in the margins of Devonian aquatic
ecosystems.

I have come across a good example of how paleontologist use character traits to classify dinosaurs. The example is short and simple and could be used - with a few modifications in a biology class.
The example comes from Thomas Holtzs’ article Chasing Tyrannosaurus and Deinonychus Around the Tree of Life: Classifying Dinosaurs
If you look at the list below you will see four dinosaurs and nine traits listed. With these nine traits we can create a simple cladogram for the four dinosaurs. Traits or characters fall into four different catagories: primitive, unique, shared derived and convergent.
If you look at the chart below, you will notice that all four dinosaurs have three traits in common: hinge in lower jaw, wishbone and bipedality. These are considered the primitive traits that are found in the common ancestor of all four dinosaurs (if you assume the common ancestor did not have the three traits then each would have had to evolve independently in all four dinosaurs - which is a much more complicated scenario). Unfortunately, the three primitive traits do not tell us much, so lets look at unique traits. There are two unique traits in the list (retractable sickle claw and backward pointing pubis) found only in Deinonychus. While these distinguish Deinonychus they don’t help us learn which of the other three dinosaurs was the closest relative of Deinonychus. So we seem to be at an impass.

Allosaurus Deinonychus Albertosaurus Tyrannosaurus
Hinge in lower jaw Yes Yes Yes Yes
Wishbone Yes Yes Yes Yes
Bipedal Yes Yes Yes Yes
Retractable sickle claw No Yes No No
Backawards-pointing pubis No Yes No No
Number of fingers 3 3 2 2
Third metatarsal in foot Unpinched Unpinched Pinched Pinched
Astragalus (ankle bone) Short Tall Tall Tall
Tip of ischium Expanded Pointed Pointed Pointed

Let’s look at the chart again. There are several traits on the list that are shared between two or more - but not all - of the dinosaurs. These are two fingers (rather than three) on the hand, pinched third metatarsal (rather than unpinched), tall astragalus (rather than short) and pointed ischium (rather than expanded). None of these traits occur in Allosaurus (or meat eaters previous to Allosaurus). At this point we can say that the condition seen in Allosaurus is the ancestral condition (although this does not mean Allosaurus was the actual ancester of the other three). At this point then, we can say we have two clade, one composed of Allosaurus and the other composed of the other three dinosaurus:

This leaves the relationships between Deinonychus, Albertosaurus and Tyrannosaurus to work out. There are three posibilities. First, Deinonychus could be more closely related to Albertosaurus. Second, Deinonychus could be more closely related to Tyrannosaurus. Third, Tyrannosaurus is more closely related to Albertosaurus. Lets go back and look at our trait list. We have determined that the hinge in the lower jaw, the wishbone and bipedality are primitive traits shared by all four dinosaurs. We have also determined the tall astragalus and pointed tip of the ischium evolved after the lineages for Deinonychus, Albertosaurus and Tyrannosaurus split from the Allosaurus lineage. We have also determined that the retractable claw and the backward pointing pubis separate Deinonychus from the others. Let’s look at the second option above (that Deinonychus is more closely related to Tyrannosaurus).

In order for this to be the case 11 evolutionary changes would have had to take place: one change for the three primitive features shared by all, one for each of the unique traits in Deinonychus and 2 for the remaining two traits in Albertosaurus and Tyrannosaurus (which would have evolved independently in each lineage). There is a simpler explanation - one involving fewer evolutionary changes. This is the third option listed above, that Albetosaurus and Tyrannosaurus are more closely related to each other which requires only nine evolutionary changes, which I will leave to the reader to work out (consult the article by Holtz, linked to above, for a more detailed explanation and better cladograms).

The Eocene saw the rise of the euprimates (a term coined by Elwyn Simons). An alternative term is “primates of modern aspect”. Going back to R. D. Martin’s definition of fossil primates the euprimates share claws replaced by nails, opposable hallux (big toe) and postorbital closure (among other things) in common with all modern primates.The euprimates are divided into two families, the adapidae and omomyidae. The adapidae are composed of two subfamilies: nothartinae (5 genera) and adapinae (14 genera). The omomyidae are divided into three subfamilies: anaptomorphinae (15 genera), omomyinae (12 genera) and microchoerinae (4 genera) Plus two species (Arapahovius and Loveina) of uncertain affinities (for those who know something of taxonomy they are Omomyidae incerta sedis).

In general, Omomyids are tarsierlike in their morphology. For example, the dental formula (which gives the number of each type of tooth) is often similar (in particular the lower dental formula is sometimes 1 incisor, 1 canine, 3 premolars and 3 molars - as seen in tarsiers). The crania have tapered snouts and the ectotympanic bone is tubular - as in tarsiers. The nasal and olfactory regions are diminished in size and the eye orbits are expanded.

The adapids, on the other hand, are generally considered to be related to lemurs. They have a divergent hallux, flattened nails on the digits. Some (such as notharctus)have a postorbital bar and a petrosal bulla. Encephalization quotients (a ration of brain and body weight) have been calculated ad are withing the range of other Eocene mammals and are slightly lower than say, the Oligocene anthropoid Aegyptopithecus. In the middle ear region they bear some resemblance to lemurs. The ectotympanic bone (which supports the tympanic membrane) is variable in adapids ranging from a free ringlike structure (as in lemurs) to one that is expanded to form part of the lateral bull wall (as in lorises and Aegyptopithecus).

The morphology of both leads to several interesting problems. First, what are the phylogenetic relationships? There are three competing theories.

1) Omomyids share some characteristics with plesiadapiformes and at one time the plesiadapiformes were thought to have given rise to omoyids. In this theory the adapids were considered ancestral to anthropoids and the prosimians (other than tarsiers).

2) Since no shared derived characters link tarsiers and anthropoids to the other prosimians it has been suggested that plesiadapiformes gave rise to the euprimates which split into two branches. One branch was composed of adapids, lemurs and lorises, the other was composed of omomyids, tarsiers and anthropoids. In this theory, tarsiers are more closely related to omomyids than to anthropoids.

3)A variant of number 2, except tarsiers are more closely related to anthropoids than they are to omomyids.

There is a further complication. In both 2 and 3 above tarsiers are grouped with anthropoids and adapids are grouped with lemurs and lorises. The problem is adapids share quite a few traits with anthropoids, tarsiers share some traits with anthropoids but not lemurs and lorises. Paleontological data supported a linking of adapids and anthropoids. Comparitive anatomy (hemochorial placenta, presence of a retinal fovea, for example) and biochemical data supported a relationship between tarsiers (and consequently omomyids) and anthropoids. This led to something of a stalemate. If tarsiers (and hence omomyidae) were more closely related to the anthropoids (as the anatomical and biochemical data suggested) then adapids (as the paleontological data suggested) couldn’t be. Which was right. A very intersting solution to this problem was presented by Gingerich and Schoeninger in 1977. The suggestion wasn’t paid much attention to until 1986, when Rasmussen (in his 1986 paper “Anthropoid Origins: A Possible Solution to the Adapidae-Omomyidae Paradox”) revived it. Grant the paleontological evidence that relates the omomyidae to tarsiers and adapidae to anthropoids. Lemurs and lorises would then form a sister group to both the omomyidae-tarsier group and the adapidae-anthropoid group. Consequently, tarsiers would be more closely related to anthropoids than to lemurs and lorises - which satifies the anatomical and biochemical evidence and the omomyid-tarsier and adapid-anthropoid groups could still be kept - satisfying the paleontological evidence. It’s a good theory, unfortunately, one small fact stands in the way. This is the traits which seem to relate adapids to lemurs and lorises. In this theory, the traits relating adapids to lemurs and lorises are due to parallel evolution. Which has raised some objections since parsimony requires little or no parallel evolution.
It’s been my experience that these kinds of situations come up a lot in primate - and human - evolution. No matter what phylogenies you create parallel evolution always comes into play. Personally, I consider a certain amount of parallel evolution to be a fact of life in primate evolution.

I examined several definitions of the order primates in a previous post and looked at how any definition of primates has to be constricted when applied to the fossil record. In particular, the closer one gets to the common ancestor between primates and other mammals (insectivores for example) the harder it becomes to tell what is a primate and what is not.

Currently, there are four theories as to which group primates arose from (although, there is general agreement that thier orgins lie in the Order Insectivora):

1) Erinoaceomorpha (hedgehogs)
2) The suborder Lipotyphla - which contains shrews, moles, tenrecs and selenodons.
3)Tree shrews
4) Archaic taxa such as Leptictidae or Apatemyidae - which may or may not be insectivores.
The earliest identifiable primates are the plesiadapiformes. The plesiadapiformes occur from the mid Paleocene to the Eocene (from about 65 mya to about 53 mya). They inhabited both North America and Europe. The plesiadapiformes are an infraorder composed of six families and almost forty genera. The families are:

1) Plesiadapidae (five genera)
2) Carpolostidae (three genera)
3) Saxonellidae (one genus)
4) Microsyopidae (24 genera)
5) Paromomyidae (three genera)
6) Picrodontidae (two genera)

The phylogeny of most of these groups has been worked out in greater or lesser detail. Consider the Plesiadapidae. The earliest genus was Pronothodectes. In North America the earliest member of this genus was Pro. matthewi. Pro. matthewi gave rise to Pro. jepi. From here it gets complicated. Pro. jepi gave rise to two different groups, Nanodectes and Plesiadapis. The Nanodectes lineage goes as follows: N. intermedius, N. gazini, N. simpsoni, and N. gidleyi. The Plesiadapid branch goes as follows: Ples. praecursor, Ples anceps. Ples anceps gave rise to two lineages. The first goes: Chiromyoides minor, C. caesor, C. potior, C. major. The second lineage of Ples. anceps goes as follows: Ples. res, Ples. churchilli. Ples. churchilli also split into two lineages. The first goes: Ples. fodinatus, Ples. dubius. The second lineage is Ples. simonsi, Ples. cookei. The phylogeny of the other families is equally complicated.

Comparitive work on modern primate dentitions has allowed us to come up with some general guidelines on determining things like diet. Based on this we say that the plesiadapiformes were primarily insectivorous with diets resembling modern prosimmians such as lemurs and galagos. However by the late Paleocene - Early Eocene some plesiadapiformes were adopting diets of fruit or leaves.

How are plesiadapifomes related to later primates? There are actually three different views on this question. One view is that plesiadapiformes are not primates because they are distinct from later primates. A second view places them in a suborder with tarsiiformes (because both groups share some traits in common - enlarged, protruding incisors, similar configurations of inner ear anatomy among others). A third view is that they are the earliest primate radiation (because they have primatelike teeth that make them important for understanding primate origins).

For Further Reading:

Primate-like mammals:
A stunning diversity in the tree tops

Archonta (primates, bats, tree shrews and flying lemurs)

The pictures below are all of primates.

The jury is still out as to whether the tree shrew (pictured below) is a primate, or is related to the primates.

Primates come in all shapes and sizes, so the question “how do anthropologists and paleontologists” is a natural one. The question necessarily involves a great deal of comparitive anatomy but I have tried to keep it to a minimum. Readers who need an overview can read this article on the anatomy of the skull. I have written this post in the form of a question and answer session.

What is a primate?
That is actually a good question and the answer is quite complicated.

What do you mean? Aren’t monkeys monkeys?
Well yes, but it’s more complicated than that. It is easier to define modern living primates. Primates as a group do share some unique, universal (among primates) features not shared by other mammals. Unfortunately, the also share features in common with other mammals.

Could you give an example of a unique (or diagnostic) feature that separates one mammal group from another?
Sure! The double pulley configuration of the astralgus (a bone in the hind limb) is diagnostic of artiodactyls.

But, what about primates?
That is a good question. Mivart first defined the order primates. His defination was:

Unguiculate, claviculate placental mammals, with orbits encircled by bone, three kinds of teeth, at least at one time of life; brain always with a posterior lobe and calcerine fissure; the innermost digit of at least one pair of extremities opposable; hallux with a flat nail or none; a well developed caecum; penis pendulous; testes scrotal; always two pectoral mammae.

Wow, that’s a lot!
Actually there is more. Mivart gave his definition in 1873. In 1959 Le Gros Clark added to it:

Preservation of generalised limb structure with primitive pentadactyly (five fingers). Enhancement of free mobility of the digits, especially of the pollex and hallux (both used for grasping). Replacement of sharp, compressed claws by flat nails; development of verysensitive tactile pads on the digits. Progressive shortening of the snout. Elaboration of the visual apparatus, with development of varying degrees of binocular vision. Reduction of the olfactory apparatus. Loss of certain elements of the primitive mammalian dentition. Preservation of a simple molar cusp pattern. Progressive expansion and elaboration of the brain especially of the cerebral cortex. Progressive and increasingly efficient development of gestational processes.

That seems pretty thorough. Is there more?
Yes, there is.

I was afraid of that.
Please don’t interupt. In 1967 Napier added two more:

Prolongation of postnatal life periods. Progressive development of truncal uprightness leading to a facultative bipedalism.

That’s a lot of information, where did you get it?
Mainly from R. D. Martin’s paper “Primates: A Definition”

So what’s the problem with the above definition of primates?
There are two problems. First, some of the above are actually trends, some of which are not features. Instead they refer to developments found only in some members of the group (remember, we are not trying to trace ancestor-descendent relationships at this point. We are trying to provide a definition of an order of mammals). Second, some of these are either traits that are probably primitive features of placental mammals or have arisen by convergence.

So, then how do we define primates?
Martin choose to examine living primates with an eye to creating a new definition, which I won’t bore you with since it is rather long.

You said the definition applies to living primates, what about fossils?
For fossils the definition has to be modified somewhat, but first we have to talk about tree-shrews.

?Tree-shrews?
Yes, you see Le Gros Clark argued that the Tupaiidae are more closely related to primates than to any other placental mammal and should be included in the order primates. This has been argued about ever since. Martin used tree-shrews as a test case for his definition of primates and decided (correctly, I think) that they were not primates.

That’s a pretty scientific approach!
Yes, it is. Paleoanthropology has a well developed scientific methodology and a rich body of theory to draw on.

So what about the fossils?
Since we have only skeletons to examine the definition has to be contracted somewhat. This is what is left:

Well developed, divergent hallux with flat terminal phalanx in the foot. Elongated distal segment of the calcaneus. Relatively large, convergent orbits with restricted interorbital distance. Postorbital bar present; ethmoid exposure in the orbit possible (depending in interorbital distance relative to skull size). Petrosal bulla. Relativly large braincase. Sylvian sulcus on endocast. Dental formula maximally 2.1.3.3/2.1.3.3. Premaxilla short; upper incisors arranged more trnsversly than longitudally. Molars with low, rounded cusps. Lower molars with raised, enlarged talonids.

So, does it identify fossil primates?
Yes, it does. According to this definition omomyids and adapids, for example, are primates.

What about plesiadapids?
The jury is still out on this issue. But see the next post.

For Further Reading:

Primate Adaptation and Evolution”

Primate Evolution: An Introduction to Mans Place in Nature

Primate Evolution

The Evolution of Primate Behavior

Major Topics in Primate and Human Evolution

Additionally, if you have access to a good (University) library:

The American Journal of Physical Anthropology
The Journal of Human Evolution
Folia Primatologica

Ichthyostega Hindlimb

Ichthyostega Skull

The above are fossils of Ichthyostga - one of the first land dwelling vertebrates. They evolved in the Devonian (some 410-360 mya)and are found in Greenland. A recent study - published in the most recent issue of Nature - indicates some interesting facts about Ichthyostega (from National Geographic News):

The team’s reconstruction differs from all previously published reconstructions of the animal.

Unlike in other reconstructions, the vertebrae that make up the backbone in Ahlberg’s rendering are regionalized: They have different shapes in different parts of the column. Therefore, different parts of the backbone flexed in different ways, Ahlberg speculates.

The shapes of the vertebrae would have prevented Ichthyostega from sideways movement. The vertebrae generally resemble those of mammals, suggesting that this part of the backbone could flex vertically to some extent, Ahlberg said.

While regionalization of the backbone is fairly common in living land vertebrates, it’s not seen in the lobe-finned fishes from which Ichthyostega is thought to have evolved. Lobe-finned fishes have thick, fleshy fins, as opposed to the delicate fins of most fish. Only two types of lobe-finned fishes survive today, coelacanths and lungfishes.

This has interesting implications for how Ichthyostega moved:

As such, the researchers hypothesize that Ichthyostega probably used two different gaits on land, depending on how fast it needed to move.

“On the one hand it could have ‘walked’ with the body held rigid and the limbs moving in [an] alternating diagonal sequence,” Ahlberg wrote in an e-mail to National Geographic News.

In this gait the strong front limbs likely allowed the creature to hold its body off the ground, while the flipperlike hind limbs and rear end dragged behind, Ahlberg noted.

In the other, inchworm-like gait Ichthyostega likely used the limited up-and-down movement of the backbone in combination with symmetrical limb movement “to achieve a weird gait approximating to a slow and extremely stumpy-legged gallop,” Ahlberg said.

Since most paleontologists assume that land vertebrates evolved from an organism that could flex their vertebral column from side to side, this means Ichthyostega probably wasn’t a direct ancestor of later vertebrates:

In other words, Ichthyostega’s body design was a failure. Few, if any, fossils representing descendants of this lineage are known after about 360 million years ago, Carroll noted in a commentary on this research in Nature. The creatures, it seemed, simply died out.

“Remember, the origin of land vertebrates from fish took 15 million years,” Carroll said in a telephone interview. That’s a long time, he added, for lobe-finned fish to have evolved various designs—with varying degrees of success.

Ahlberg said that another Devonian tetrapod from Greenland, Acanthostega, which is more primitive and less terrestrial looking than Ichthyostega, appears closer to the “main line” of tetrapod evolution. Below is a fossil Acanthostega.

For more info you can go to:

Palaeos

Tree of Life

DarkSyd at Unscrewing The Inscrutable has sent me another post. This time it is on the evolution of birds.

***************************************************************

OH! I have slipped the surly bonds of earth, and danced the skies on laughter’s silvered wings
Sunward I’ve climbed and joined the tumbling mirth of sun-split clouds
And done a hundred things you have not dreamed of- wheeled and soared and swung high on the sunset silence …

Who among us has not dreamt of flight? To coast effortlessly along on lazy thermals on a sunny day, the distant tapestry far below painted in pastel blues, browns, and greens? To dive at the ground at over 150 MPH and then pull out and blaze over a blurred landscape at breathtaking speed? From Daedalus to Da Vinci to Apollo and beyond, it is a vision that has driven mankind. The idea of flying informs our dreams, infiltrates our legends, and invades our nightmares. And now that we can fly by way of machines, it turns out to be every bit exciting as we imagined. But why are the birds heir to this ability? Why and how were they chosen to be among the lucky few who would take to the air and go on to dominate the land, sea, and sky?

Huge Grahics Below!

The most famous fossil ever found was unearthed with no ceremony in an old Bavarian stone quarry, a site known for high quality flat limestone rock called Plattenkalk since the days of Rome. It was at first a curiosity, later a sensation, and now reigns supreme among them all. It is so detailed it quickly earned the name Lithographica; lithos as in Latin for stone, and graphos as in Greek for writing. The new find was a volume literally written in stone.

The specimen would have been a star on its own. But because of the year it was discovered, 1861 when Darwin’s Dangerous idea is in full bloom having taken the scientific community by storm only two years earlier, it was propelled to fossil superstardom. The slab would become the most famous specimen preserved in the ancient medium of rock ever unearthed. But it harbored a secret kept hidden for a hundred years or more after it was dug out of the ground: The dinosaurs may not have gone extinct at all.

It looked like a lizard, down to the teeth and the tail, yet had some of the bones of a bird. And just case there was any doubt, fully functional flight feathers extending off of what look like short wings are preserved in the limestone matrix. It was called Archaeopteryx , meaning primitive wing. And it remains the oldest bird ever found.

                               

Archaeopteryx lithographica from the Late Jurassic Solnhofen limestones of Germany. The photo above is of the real deal; the original fossil itself. Most photographs you’ll see are of poor-quality casts. It ain’t easy for scientists to even get near this slab. It’s more valuable than diamonds

A word of caution: I have several friends who eat, breathe, and live dinosaur/bird evo AKA paleo-ornithology. They speaketh a strange language I do not understand full of morphological, anatomical, cladistics terms. But, every one of those folks is solidly behind the dinos-to-birds scenario and judges the opposing hypothesis as so bad it’s in their own words “Creationist Bad”. I’m not an expert. This piece may have some errors in it and I appreciate any pointers. But for the purposes of this article I’ve given those aforementioned dino aficionados the benefit of the doubt.

The parallels between birds and dinosaurs were not lost on early paleobiologigists. Even so eminent an evolutionary pioneer as Thomas Huxley made the case that birds might be direct descendants of dinos. But over the next few decades the Great Dinosaur Bone Wars heated up among rival fossil hunters and the emphasis shifted to bigger and meaner looking ancient beasts. Long-necked sauropods and toothy Carnosaurs soon filled the scientific literature and museum halls, and the impression of dinosaurs as big, brainless, slow moving, very unbird-like reptiles, took hold for generations.

That would not change until 1960 when Dr. J.H. Ostrom’s published an exhaustive study of Deinonychus antirrhopus illuminating similarities to Archaeopteryx. That work provided the impetus for a paradigm shift in ideas on the origin of birds and the evolution of flight.

By the time Jurassic Park rolled into cinematic history, the birds were accepted as likely descendants of a dinosaur group called Maniraptors, or ’seizing hands’, which includes the dromaeosaurids such as velicoraptors. The resemblance is clear:

               

                      Enlarge

I’m not qualified to delve more deeply into the anatomical congruencies between birds and dinosaurs and it’s a detailed subject beyond the scope of this modest effort. For those of you so inclined, this article by Chris Nedin in the Talk Origins archive called All About Archaeopteryx is a must read.

Exactly why feathers first evolved will likely never be known. They’re useful for insulation and protection, they provide a platform for coloration, and they can be used as both displays to ward off predators or attract mates. But the process of how it transpired may be coming to light, again courtesy of those wonderful fossils form China, and the relatively new field of evolutionary development.

                     

The chart above outlines a plausible evolutionary scenario. From simple stringy feathers, to more complex stringy feathers, to flight feathers. This idea is based on the two bodies of data we do have: The variety of feathers found dinos in the fossil record and how they develop in a bird embryo.

However feathers developed, it’s still a fascinating question as to how the precursors of birds learned to fly. The two possibilities are the ground up theory and the trees down theory. In the trees down theory early birds first become gliders and then refine that ability until they’re capable of powered flight. In the ground up theory the small dinosaurs flapped their feathered arms while running to provide extra power and maneuvering ability. No one knows for sure which one of these broad possibilities was the path by which birds first took to the air. But once they did, feathers offered them an advantage unavailable to their aerial competitors of the day such as pterosaurs and other flying reptiles: Feathered wings are better suited to flying in and around thorny trees and bushes. If a wing of stretched skin tears on close encounter with a plant, that animals is grounded until it heals or the critter dies. Feathers aren’t subject to this design flaw.    

For years creationists worked very hard to discredit Archy. Most were fond of saying “It has true perching feet” as if this somehow was a problem for the same process of evolution which turned forelimbs into wings. Another objection to Archy and transitionals in general was somehting like “Every animals appears ‘fully formed’ in the fossil record”. I’m unsure what a half-formed bird would be excatly … a flying squirrel or a sugar glider perhaps? A deformed bird with ‘half-formed’ body parts would not last long. Natural selection would make quick, grisly work of such an unfortunate animal. Finding lots of adult birds that were deformed in the fossil record would be evidence against evolution.

When it began to soak into the laypublic that raptorial dinosaurs were thought to be the direct ancestor of modern birds, and when this idea excited young movie goers, the creationists of course went full tilt, adopting the existing scientific dissent over the Bird-Dino origin theory.

Most creationist nonsense on bird transitionals now comes from Answers in Genesis; the crew of the Discovery Institute doesn’t talk much about birds preferring to focus on PR issue centered on philosophy or areas of mystery such as the Cambrian Explosion. But to get a taste of AiG’s duplicity and see an example of how evo-devo can utterly smite them, I heartily encourage any well informed laymen to read PZ Myers’ Digit Numbering and Limb development. It’s short, well illustrated, written for the educated layman and that article by PZ is the best intro into evo-devo and dinosaur homology I have ever read in my life.

The competing idea to the dino origin of assigns birds as descendants of Thecodantia; an ancient clade of reptiles which predate dinosaurs and gave rise to what are called basal archosaurs which gave rise to crocodiles and turtles. The Thecodont Theory/Hypothesis basically says that birds aren’t derivative dinosaurs. The primary proponents are Alan Feduccia and Larry Martin.

Then in 1995 dino hunters got lucky in an obscure quarry in the Liaoning Province of China: In series of amazing finds, one after the other, a whole new window was opened up on dinosaur evolution.

The first was this fossil of a Sinosauropteryx showing what appeared to be an exterior of stringy feathers. In rapid succession, more feathered dinosaurs were found, each one more exquisite and stunning:

Sinosauropteryx fossil in situ and artistic impression

                                               

Generic raptor fossil in situ and artistic impression

 

        Caudipteryx fossil in situ and artistic impression

 

     Microraptor Gui in situ and Artist’s Impression

Some of the Chinese dinos are enigmatic, such as the four winged microraptor above; or any member of the Therizanosaurids. Theriz’s are wierd!  Most of them have these giant claws the utility of which is wide open to speculation. Scooping fish out of streams or insects out of termite mounds? We have no idea: Therizanosaurs are an excellent field of study for any of you budding paleontologists because so little is understood about these creatures. Currently Therizanosaurs are thought to be descendants of early raptors and were in the process of evolving into generalists and herbivores. If so, they plausibly had feathers, as shown below in an artists rendition.

Based on this new material, we can construct what an evolutionary progression from dinos to birds might have looked like. Again, this is not intended to represent a straight line order of ascendancy, but it does gives us a rough idea of how the transition may have occurred:

Given the new finds of feathered dinos in China, one might think the Thecodont idea would be history. But there remains one intriguing possibility: The feathered dinos of China lived more recent than archaeopteryx. That means either feathers first evolved in dinosaurs which later diversified into avian and non avian critters. Or birds did evolve from non-saurian ancestors and then later various clades of flightless birds evolved into what we think of as raptorial dinosaurs!

This Neoflightless Hypothesis is best summarized and criticized as the idea that raptors were a ‘Mesozoic Kiwi’ or ostrich. This idea is a long shot, I wouldn’t bet even odds on it. But it is possible, it is consistent with the fossil record. We know that very large predatory flightless birds have evolved from their winged ancestors on several independent occasions. (It’s also interesting that creationist use the neoflightless hypothesis to discredit evolution by way of birds and dinosaurs. Apparently a sparrow evolving into a one ton raptor is not ‘macro-evolution’ but going the other way is …)

The most recent such radiation includes the now extinct Moa of Australia along with the so-called terror birds which ruled the grasslands and forests of North and South America between 65 million years ago and just a couple of million years ago. These killer birds were three meters tall in some cases, with a skull the size of a watermelon sporting a broad beak shaped like an ax blade, and at least one of them even developed rudimentary hands! So yes, it is within the realm of possibility that birds evolved into raptorial dinosaurs.

But regardless if dinos evolved into birds or birds evolved into dinosaurs, it would be correct to say that birds are a type of dinosaur. So let’s take a look at a few of the dinosaurs among us.

           

Snowy owl                               Red Jungle Fowl, ancestor of the domestic chicken

A European Imperial Eagle guards its nest and chicks

A young Peregrine Falcon finishing off a tasty morsel. These are the fastest known animals and can reach speeds of almost 200 MPH when diving

Meet the Penguins courtesy of NATURE

An American Bald Eagle takes flight              A Kingfisher peering into a stream prepares to strike

Birds are among the most successful of all the ‘tetrapod’ vertebrates with roughly 10,000 species known or suspected. Not surprising: They can pull up stakes and fly away to remote islands and oasis no other large animal can reach, or travel across the entire globe in search of warmer climes; they’re equipped with the eyes of a surveillance satellite, the best in the entire animal kingdom; they have big brains and excellent memories; and if we humans don’t make it, given time I wouldn’t be surprised if a descendent of birds rose to sentience. Intelligent Dinosaurs: what an interesting race of beings that would be!

Perhaps one day in the not too distant future on a planet or moon of lower gravity, we too will be able to fly with the birds under our own power; to ply the alien winds as masters of the sky. Until then we can only dream of such a day and get a taste of what it might be like to soar and wheel and dance the skies on laugher’s silvered wings in our clunky aircraft; and that’s not a bad appetizer.

High in the sunlit silence. Hov’ring there, I'’ve chased the shouting wind along, and flung my eager craft through footless halls of air.
Up, up the long delirious, burning blue I'’ve topped the windswept heights with easy grace, where never lark, or even eagle flew.
And, while with silent, lifting mind I’ve trod The high untrespassed sanctity of space, Put out my hand, and touched the face of God

Caimans,

Gavials,

Alligators,

and Crocodiles

have a long and complex evolutionary history. The Crocodylia have undergone at least three separate episodes of adaptive radiation during their history. They are all members of the order Crocodylia - which in turn is a member of the superorder Archosauria. Fellow members of the Archosauria include the Saurischia (dinosaurs such as Diplodocus), Ornithischia (bird hipped dinosaurs such as Stegosaurus), the flying Pterosauria and the Thecodontia. The origins of the Crocodylia lie over 200 million years ago in the Triassic period. The earliest know genus of Crocdylia are known as Sphenosuchia. One of them, Gracilisuchus of the upper Triassic, is pictured below.

Such as the above fossil (found in Argentina). There are several features of the skeleton that distinguish Crocodylia from other reptiles. In humans, for example, three bones come together in the pelvis to form a structure called the acetabulum. The acetabulum is a cup shaped area where the femur (or thigh bone) joins the trunk. As you can see in the picture of a human pelvis below.

In the Crocodylia, however, this is not the case. The pubis is almost completely, or is completely excluded from forming part of the acetabulum. A second feature of the skeleton that distinguishes Crocodylia from other reptiles occurs in the ankle (yes, Alligators and Crocodiles have ankles). In humans, the talus sits ontop of the calcaneus.

In the Crocodylia, however, they sit side by side (note the talus is also called the astragalus).

This arrangement allows the foot to be twisted forward and allows the leg to be more underneath the body (compared to lizards). The Gracilosuchus pictured above shares these features with the Crocodylia.
One of the interesting things about the Sphenosuchia is that they were relatively long limbed and land dwelling. The Terrestrisuchus below is a good example.

The next group of Crocodylia are the Protosuchia which appeared in the lower Jurassic. Like the Sphenosuchia, the Protosuchia have long limbs and were largely land dwelling. Although we begin to see some of the adaptations to water that characterize later Crocodylia - for example the nostil cavity is becoming separated from the mouth.

The Protosuchia were distributed throughout the world.
The Mesosuchia evolved in the lower and upper Jurassic and are excellent transitional fossils between the Protosuchians and the Eusuchians (modern crocodiles, alligators, caimans and gavials). The Mesosuchians were a highly diverse group containing over 70 genera. The Mesosuchia are divided into aquatic, semi-aquatic and terrestrial species. The Teleosauridae are good examples.

In the Teleosauridae the limbs have become shorter and the snouts have become larger. Another group is the Metriorhynchidae which have become highly adapted to water living. For example their forelimbs have become transformed into paddles.

land dwelling species include Baurusuchus.

.
Semiaquatic forms were represented by the Goniopholidae and the Atoposauridae. It is from the Atoposauridae, and a species called Theriosuchus, that the modern Eusuchians are believed to have evolved.
Alligators first show up in the upper Cretaceous, Gavials in the Miocene (about 25 million years ago), Crocodiles in the Paleocene (about 60 million years ago) and Caimans in the Oligocene (about 30 million years ago).

Dave Winter over at Science and Sensibility has sent me the following post on Sea Squirts. Thanks David!

Take a close look at the picture above this text. If you haven’t
been introduced to a sea squirt before you may be surprised to learn
the blue blob depicted above is an animal. If that surprised you then
you’ll be flabbergasted to learn you are more closely related to that
animal than 97% of all the species so far described on earth and very
much more than 99% of the individual organisms here.

The animal
in question is a sea squirt (or Urochordate or Tunicate) and it fits
into the Phylum Chordata - the same one as you, me and all the
vertebrates. On the face of it that seems a puzzling designation, the
animal in the picture doesn’t look much like respectable chordates
like mammals or birds or fish, or even like visually similar
invertebrates like the lancelet. That is, until you realise that what
you’re looking at is one half of a lifecycle split into two such
disparate parts that it would do a butterfly proud. See, sea squirts
start out life as larvae that look more or less like tadpoles:

Now that’s a bit more like it. Sea squirt larvae contain notochords, nicely defined muscle segments and a
post-anal tail . Just like you. When you where an embryo. The tiny
tadpoles swim in the plankton before they metamorphose into their
adult form. When the time comes they plant themselves head first onto
some hard surface (rocks, reefs and piers being among their
favourites) and proceed in dispensing with all the stuff they needed
as a free swimming tadpole - including their tail and the head
ganglion which served the role of a brain From here the sedentary,
adult form develops. Essentially the adult sea squirt is a sack fill
of sea water with two syphons - one “sucking” water in and the other
“spitting” it out. These syphons ensure water flows over the
pharyngeal basket which filters out food.

The sea squirts are
also of interest to evolutionary biologists for a number of reasons.
Firstly they may be able to tell us something about the evolution of
the vertebrates. All the vertebrates share a common ancestor with the
sea squirts, there are two main schools of thought as to how each
lineage (vertebrates and sea squirts) got to where they are
today:

1. The common ancestor shared by sea squirts and
   other chordates was something like a sea squirt larva. After
   that point the sea squirts took up a sedentary “adult stage” while the
   vertebrates went on to form bones and limbs and all those great
   things.
2. The common ancestor shared by sea squirts and other chordates was
   something like a modern sea squirt. At some stage a sea squirt larva
   became sexually mature before it took on a filter feeding “adult”
   stage. In this scheme the individual larva that reached sexual
   maturity as a ‘tadpole’ would be the progenitor of all vertebrates.
   The development of sexual maturity in an otherwise ‘juvenile’ stage is
   called neoteny and is epitomized by the Axolotl which is a sexually
   mature salamander larva.

Richard Dawkins reports
in The Ancestor’s Tale that molecular data tends to support the
first of those schemes. Coincidentally, that scheme was proposed by
that most prescient of biologists, Charles Darwin.

The other
reason sea squirts interest evolutionary biologists is that, seen
through our vertebrate minds, they seem to be bristling with potential
for great things. Recent studies have shown that the sea squirts have
genes containing all the motifs needed to get a blood clotting system
going. Additionally they have some proteins that look a lot like
Toll receptors - proteins associated with identifying pathogens in our
system. In fact a glance through the genome of a sea
squirt revels the seeds of nervous systems, eyesight, the immune
system, and even the cardiovascular system.

Of course we
shouldn’t get carried away on thinking that the sea squirts are some
sort of museum piece displaying the seeds from which we sprung. The
sea squirts have kept what genes they have for a reason - living
organism can be primitive but they can’e be ancestral. It’s likely
that genes we share with sea squirts have been co-opted into
completely different roles and many that where in our common ancestor
500 million years ago have probably been lost. Still, not too bad for
a blue blob huh?

… if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection.
Charles Darwin, On the origin of Species, 1859¹

Another way to express what Darwin meant is that individuals with characteristics that are useful to them in their struggle to survive are more likely to produce offspring with the same characteristics. One straightforward consequence of Darwin’s idea is that animals that are sick or injured will be less successful to defend themselves against predators or to escape from them. Therefore, such individuals are more likely to be killed by predators before they have a chance to reproduce. This is so obvious that it should hardly need to be proven. Nevertheless, it has been demonstrated to take place in the wild many times. One example involving African wild dogs (Lycaon pictus) and their prey impala (Aepycerus melampus) in Zimbabwe was published recently².

African wild dogs
African wild dogs (also called African hunting dogs) hunt in packs and share their kill. One large animal that wild dogs prefer to hunt is impala. Once a pack finds an impala, or any other prey, they begin to chase it until the prey gets tired and they catch up with it. Such chases can apparently last for several kilometers at high speeds. Obviously, this hunting method is energetically very costly. Imagine yourself having to run, say, 2 kilometers (about 1.3 miles) as fast as you can before your every meal. Just to obtain enough energy to be able to run 3x2 fast kilometers a day, you would probably have to eat an extra meal every day, but that would make it necessary to run 2 additional kilometers!

This being the case, which impalas would a pack of wild dogs rather go after to minimize their energy expenditure during a hunt? Undoubtedly, the weak and the sick ones, because they will be slower than the healthier animals, and by chasing the slower impalas the wild dogs will spend less energy.

Impalas in Africa

To determine if this was indeed what was happening in the wild, British scientists² collected bone marrow from one group of impalas that had been killed by wild dogs and another group that had been killed non-selectively by humans. They knew from previous studies that impalas in poor condition had very little fat in their bone marrows. Therefore, as a measure of the physical condition of each impala they calculated the amount of fat it had in its bone marrow at the time of its death. The graph below shows their results.

The impalas that had been killed by wild dogs (bottom curve) had significantly less marrow fat than the impalas that had been killed by humans (top curve). Therefore, the authors of the study concluded that wild dogs selectively prey on impalas that are in poorer condition.

A more general result that we can derive is that the weaker impalas are less likely to live long enough to reproduce, while the stronger ones are more likely to escape from wild dogs (and other predators) and live long enough to reproduce. What does this mean in terms of evolution? It means that the healthier and stronger impalas are more likely to pass the genes that contribute to their good health and physical strength onto their offspring. In turn, their offspring will be more likely to be healthy and so on. This is basically how natural selection works.

Dinner is being served

For more information follow these links.

Natural selection 1
Natural selection 2
African wild dog 1
African wild dog 2
Impala 1
Impala 2

1. Charles Darwin, On the Origin of Species, 1859. full text
2. Alistair Pole, Iain J. Gordon and Martyn L. Gorman. African wild dogs test the ’survival of the fittest’ paradigm. Proc. R. Soc. Lond. B (Suppl.) Biology Letters 270, S57 (2003).

Wild dog and impala pictures were downloaded from the University of Michigan Museum of Zoology Animal Diversity Web

Cross-posted at Snail’s Tales